Many industrial processes require precise control of various process fluids. For example, in the pharmaceutical and semiconductor industries, mass flow controllers are used to precisely measure and control the amount of a process fluid that is introduced to a process tool. A fluid can be any type of matter in any state that is capable of flow such as liquids, gases, and slurries, and comprising any combination of matter or substance to which controlled flow may be of interest.
Conventional mass flow controllers (MFCs) generally include four main portions: a flow meter, a control valve, a valve actuator, and a controller. The flow meter measures the mass flow rate of a fluid in a flow path and provides an electrical signal indicative of that flow rate. Typically, the flow meter may include a mass flow sensor and a bypass. The mass flow sensor measures the mass flow rate of fluid in a sensor conduit that is fluidly coupled to the bypass. The mass flow rate of fluid in the sensor conduit is related to the mass flow rate of fluid flowing in the bypass, with the sum of the two being the total flow rate through the flow path controlled by the mass flow controller.
FIG. 1 shows schematically a typical mass flow controller 100 that includes a block 110, which is the platform on which the components of the MFC are mounted. A thermal mass flow meter 140 and a valve assembly 150 containing a valve 170 are mounted on the block 110 between a fluid inlet 120 and a fluid outlet 130. The thermal mass flow meter 140 includes a bypass 142 through which typically a majority of fluid flows and a thermal flow sensor 146 through which a smaller portion of the fluid flows.
Thermal flow sensor 146 is contained within a sensor housing 102 (portion shown removed to show sensor 146) mounted on a mounting plate or base 108. Sensor 146 is a small diameter tube, typically referred to as a capillary tube, with a sensor inlet portion 146A, a sensor outlet portion 146B, and a sensor measuring portion 146C about which two resistive coils or windings 147, 148 are disposed. In operation, electrical current is provided to the two resistive windings 147, 148, which are in thermal contact with the sensor measuring portion 146C. The current in the resistive windings 147, 148 heats the fluid flowing in measuring portion 146 to a temperature above that of the fluid flowing through the bypass 142. The resistance of windings 147, 148 varies with temperature. As fluid flows through the sensor conduit, heat is carried from the upstream resistor 147 toward the downstream resistor 148, with the temperature difference being proportional to the mass flow rate through the sensor.
An electrical signal related to the fluid flow through the sensor is derived from the two resistive windings 147,148. The electrical signal may be derived in a number of different ways, such as from the difference in the resistance of the resistive windings or from a difference in the amount of energy provided to each resistive winding to maintain each winding at a particular temperature. Examples of various ways in which an electrical signal correlating to the flow rate of a fluid in a thermal mass flow meter may be determined are described, for example, in commonly owned U.S. Pat. No. 6,845,659, which is hereby incorporated by reference. The electrical signals derived from the resistive windings 147,148 after signal processing comprise a sensor output signal.
The sensor output signal is correlated to mass flow in the mass flow meter so that the fluid flow can be determined when the electrical signal is measured. The sensor output signal is typically first correlated to the flow in sensor 146, which is then correlated to the mass flow in the bypass 142, so that the total flow through the flow meter can be determined and the control valve 170 can be controlled accordingly. The correlation between the sensor output signal and the fluid flow is complex and depends on a number of operating conditions including fluid species, flow rate, inlet and/or outlet pressure, temperature, etc.
The process of correlating raw sensor output to fluid flow entails tuning and/or calibrating the mass flow controller and is an expensive, labor intensive procedure, often requiring one or more skilled operators and specialized equipment. For example, the mass flow sensor may be tuned by running known amounts of a known fluid through the sensor portion and adjusting certain signal processing parameters to provide a response that accurately represents fluid flow. For example, the output may be normalized, so that a specified voltage range, such as 0 V to 5 V of the sensor output, corresponds to a flow rate range from zero to the top of the range for the sensor. The output may also be linearized, so that a change in the sensor output corresponds linearly to a change in flow rate. For example, doubling of the fluid output will cause a doubling of the electrical output if the output is linearized. The dynamic response of the sensor is determined, that is, inaccurate effects of change in pressure or flow rate that occur when the flow or pressure changes are determined so that such effects can be compensated.
A bypass may then be mounted to the sensor, and the bypass is tuned with the known fluid to determine an appropriate relationship between fluid flowing in the mass flow sensor and the fluid flowing in the bypass at various known flow rates, so that the total flow through the flow meter can be determined from the sensor output signal. In some mass flow controllers, no bypass is used, and the entire flow passes through the sensor. The mass flow sensor portion and bypass may then be mated to the control valve and control electronics portions and then tuned again, under known conditions. The responses of the control electronics and the control valve are then characterized so that the overall response of the system to a change in set point or input pressure is known, and the response can be used to control the system to provide the desired response.
When the type of fluid used by an end-user differs from that used in tuning and/or calibration, or when the operating conditions, such as inlet and outlet pressure, temperature, range of flow rates, etc., used by the end-user differ from that used in tuning and/or calibration, the operation of the mass flow controller is generally degraded. For this reason, the flow meter can be tuned or calibrated using additional fluids (termed “surrogate fluids”) and or operating conditions, with any changes necessary to provide a satisfactory response being stored in a lookup table. U.S. Pat. No. 7,272,512 to Wang et al., for “Flow Sensor Signal Conversion,” which is owned by the assignee of the present invention and which is hereby incorporated by reference, describes a system in which the characteristics of different gases are used to adjust the response, rather than requiring a surrogate fluid to calibrate the device for each different process fluid used.
Control electronics 160 control the position of the control valve 170 in accordance with a set point indicating the desired mass flow rate, and an electrical flow signal from the mass flow sensor indicative of the actual mass flow rate of the fluid flowing in the sensor conduit. Traditional feedback control methods such as proportional control, integral control, proportional-integral (PI) control, derivative control, proportional-derivative (PD) control, integral-derivative (ID) control, and proportional-integral-derivative (PID) control are then used to control the flow of fluid in the mass flow controller. A control signal (e.g., a control valve drive signal) is generated based upon an error signal that is the difference between a set point signal indicative of the desired mass flow rate of the fluid and a feedback signal that is related to the actual mass flow rate sensed by the mass flow sensor. The control valve is positioned in the main fluid flow path (typically downstream of the bypass and mass flow sensor) and can be controlled (e.g., opened or closed) to vary the mass flow rate of fluid flowing through the main fluid flow path, the control being provided by the mass flow controller.
In the illustrated example, the flow rate is supplied by electrical conductors 158 to a closed loop system controller 160 as a voltage signal. The signal is amplified, processed and supplied to the control valve assembly 150 to modify the flow. To this end, the controller 160 compares the signal from the mass flow sensor 140 to predetermined values and adjusts the proportional valve 170 accordingly to achieve the desired flow.
FIG. 2 illustrates a schematic block diagram of a typical mass flow controller 200. The mass flow controller illustrated in FIG. 2 includes a flow meter 210, a Gain/Lead/Lag (GLL) controller 250, a valve actuator 260, and a valve 270.
The flow meter 210 is coupled to a flow path 203. The flow meter 210 senses the flow rate of a fluid in the flow path, or in a portion of the flow path, and provides a raw flow signal indicative of the sensed flow rate. The raw flow signal is typically conditioned, that is, it is normalized, linearized, and compensated for dynamic response. A conditioned flow signal FS2 is provided to a first input of GLL controller 250. The conditioned flow signal FS2 is also provided to a signal filter 220, which provides appropriate signal levels as input to a display 225, which displays the flow rate to an operator.
In addition, GLL controller 250 includes a second input to receive a set point signal SI2. A set point refers to an indication of the desired fluid flow to be provided by the mass flow controller 200. The set point signal SI2 may first be passed through a slew rate limiter or filter 230 prior to being provided to the GLL controller 250. Filter 230 serves to limit instantaneous changes in the set point in signal SI2 from being provided directly to the GLL controller 250, such that changes in the flow take place over a specified period of time. It should be appreciated that the limiter or filter 230 may be omitted, and that any of a variety of signals capable of providing indication of the desired fluid flow is considered a suitable set point signal. The term set point, without reference to a particular signal, describes a value that represents a desired fluid flow.
Each of the components of MFC 200 has an associated gain, which gains can be combined to determine a system gain. In block 240, a reciprocal gain term G is formed by taking the reciprocal of a system gain term and applying it as one of the inputs to the GLL controller. It should be appreciated that the reciprocal gain term may be the reciprocal of all or fewer than all of the gain terms associated with the various components around the control loop of the mass flow controller. For example, improvements in control and stability may be achieved by forming the reciprocal of the product of the individual component gain terms. However, in preferred embodiments, gain term G is formed such that the loop gain remains a constant (i.e., gain G is the reciprocal of the system gain term).
Pressure sensed at the inlet 208 or elsewhere provides a pressure signal 290 to flow meter 210 to compensate for spurious indications due to pressure transients. Further, the pressure signal may be used by GLL controller 250 for feed forward control of the valve. Also, the pressure signal may be used to adjust the gain in a GLL controller.
Based in part on the flow signal and the set point signal SI2, the GLL controller 250 provides a drive signal DS to the valve actuator 260 that controls the valve 270. The valve 270 is typically positioned downstream from the flow meter 210 and permits a certain mass flow rate depending, at least in part, upon the displacement of a controlled portion of the valve 270. The controlled portion of the valve 270 may be a moveable plunger placed across a cross-section of the flow path 203. The valve 270 controls the flow rate in the fluid path by increasing or decreasing the area of an opening in the cross section where fluid is permitted to flow. Typically, mass flow rate is controlled by mechanically displacing the controlled portion of the valve by a desired amount. The term displacement is used generally to describe the variable of a valve on which mass flow rate is, at least in part, dependent. As such, the area of the opening in the cross section is related to the displacement of the controlled portion, referred to generally as valve displacement.
The displacement of the valve is often controlled by a valve actuator, such as a solenoid actuator, a piezoelectric actuator, a stepper actuator etc. In FIG. 2, valve actuator 260 is a solenoid type actuator; however, the present invention is not so limited, as other alternative types of valve actuators may be used. The valve actuator 260 receives drive signal DS from the controller and converts the signal DS into a mechanical displacement of the controlled portion of the valve. Ideally, valve displacement is purely a function of the drive signal. However, in practice, there may be other variables that affect the position of the controlled portion of the valve.
When the input pressure changes, for a brief period of time the sensor output does not accurately indicate the mass flow. To mitigate this effect, some mass flow controllers include a pressure transducer. Pressure transducers allow tuning of the dynamic response of the device as a function of pressure, which in turn can provide a faster response, especially at low inlet pressures. For example, U.S. Pat. No. 7,273,063 to Lull et al., which is commonly owned with the present application and which is hereby incorporated by reference, uses signals from a pressure transducer to modify the sensor signal to compensate for some pressure related transient effects and provides some compensation for changes in the amount of gas in the inventory volume.
There is some unavoidable internal volume between the flow meter and the control valve. That volume, referred to as an “inventory volume,” (e.g. 140 of FIG. 1, and 280 of FIG. 2) contains a small amount of gas that varies with pressure and temperature. An inventory volume exists between the flow meter and any downstream restriction, with the control valve being an example of a restriction. As the input pressure to the flow meter changes, a certain net amount of fluid flows into or out of the inventory volume to equalize the inventory volume pressure with that of the rest of the system, thus changing the amount, that is the mass, of fluid stored in that inventory volume. When input or output pressure changes, there is a net flow into or out of the inventory volume, and this leads to a discrepancy between the flow through the flow meter and the flow actually delivered to the process. U.S. Pat. No. 7,273,063 compensates for this by simply differentiating the inlet pressure and subsequently applying a filter, for example, to generate a transient compensating signal that nominally matches that spike for the signal inside the flow meter. The transient compensating signal is subtracted from the signal from the flow meter to compensate for the pressure change. The technique of U.S. Pat. No. 7,273,063 provides accurate sensor output for some gases, but does not provide sufficiently accurate signals for other gases.
While the presence of the inventory volume is known and attempts have been made to compensate for the volume to properly indicate flow, present methods are insufficiently accurate for the increasingly demanding standards of industry.